RFC 4335 SSH Break Extension January 20061. Introduction
The Secure Shell (SSH) [5] session channel provides a mechanism for
the client-user to interactively enter commands and receive output
from a remote host while taking advantage of the SSH transport's
privacy and integrity features. SSH is increasingly being used to
replace Telnet for terminal access applications.
A common application of the Telnet protocol is the "Console Server"
[7] whereby a Telnet Network Virtual Terminal (NVT) can be connected
to a physical RS-232/V.24 asynchronous port, making the Telnet NVT
appear as a locally attached terminal to that port, and making that
physical port appear as a network-addressable device. A number of
major computer equipment vendors provide high-level administrative
functions through an asynchronous serial port and generally expect
the attached terminal to be capable of sending a BREAK signal.
A BREAK signal is defined as the TxD signal being held in a SPACE
("0") state for a time greater than a whole character time. In
practice, a BREAK signal is typically 250 to 500 ms in length.
The Telnet protocol furnishes a means to send a "BREAK" signal, which
RFC 854 [1] defines as "a signal outside the USASCII set which is
currently given local meaning within many systems". Console Server
vendors interpret the TELNET BREAK signal as a physical BREAK signal,
which can then allow access to the full range of administrative
functions available on an asynchronous serial console port.
The lack of a similar facility in the SSH session channel has forced
users to continue the use of Telnet for the "Console Server"
function.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [2].
The "byte", "boolean", "uint32", and "string" data types are defined
in [3].
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RFC 4335 SSH Break Extension January 20063. The Break Request
The following channel-specific request can be sent over a session
channel (as described in [4]) to request that the remote host perform
a BREAK operation.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string "break"
boolean want_reply
uint32 break-length in milliseconds
If the BREAK length cannot be controlled by the application receiving
this request, the BREAK length parameter SHOULD be ignored and the
default BREAK signal length of the chipset or underlying chipset
driver SHOULD be sent.
If the application receiving this request can control the BREAK
length, the following suggestions are made regarding BREAK duration.
If a BREAK duration request of greater than 3000 ms is received, it
SHOULD be interpreted as a request for a 3000 ms BREAK. This
safeguard prevents an unreasonably long BREAK request from causing a
port to become unavailable for as long as 49.7 days while executing
the BREAK. Applications that require a longer BREAK may choose to
ignore this suggestion. If BREAK duration request of less than 500
ms is received, it SHOULD be interpreted as a 500 ms BREAK since most
devices will recognize a BREAK of that length. Applications that
require a shorter BREAK may choose to ignore this suggestion. If the
BREAK length parameter is 0, the BREAK SHOULD be interpreted as the
default BREAK signal length of the chipset or underlying chipset
driver. If no default exists, 500 ms can be used as the BREAK
length.
If the SSH connection does not terminate on a physical serial port,
the BREAK indication SHOULD be handled in a manner consistent with
the general use of BREAK as an attention/interrupt signal; for
instance, a service processor that requires an out-of-band facility
to get the attention of a system it manages.
In a case where an SSH connection cascades to another connection, the
BREAK SHOULD be passed along the cascaded connection. For example, a
Telnet session from an SSH shell should carry along an SSH-initiated
BREAK, and an SSH client initiated from a Telnet connection SHOULD
pass a BREAK indication from the Telnet connection.
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RFC 4335 SSH Break Extension January 2006
If the 'want_reply' boolean is set, the server MUST reply using an
SSH_MSG_CHANNEL_SUCCESS or SSH_MSG_CHANNEL_FAILURE [5] message. If a
BREAK of any kind was preformed, SSH_MSG_CHANNEL_SUCCESS MUST be
sent. If no BREAK was preformed, SSH_MSG_CHANNEL_FAILURE MUST be
sent.
This operation SHOULD be supported by any general purpose SSH client.
4. Security Considerations
Many computer systems treat serial consoles as local and secured, and
interpret a BREAK signal as an instruction to halt execution of the
operating system or to enter privileged configuration modes. Because
of this, extra care should be taken to ensure that SSH access to
BREAK-enabled ports are limited to users with appropriate privileges
to execute such functions. Alternatively, support for the BREAK
facility MAY be implemented as configurable on a per-port or
per-server basis.
Implementations that literally interpret the BREAK length parameter
without imposing the suggested BREAK time limit may cause a denial of
service to or unexpected results from attached devices receiving the
very long BREAK signal.
5. IANA Considerations
IANA has assigned the Connection Protocol Channel Request Name
"break" in accordance with [6].
6. References6.1. Normative References
[1] Postel, J. and J. Reynolds, "Telnet Protocol Specification",
STD 8, RFC 854, May 1983.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol
Architecture", RFC 4251, January 2006.
[4] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
[5] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Connection Protocol", RFC 4254, January 2006.
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RFC 4335 SSH Break Extension January 2006
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